The present invention relates to a thin-film circuit with integrated resistors and conductors and to a method of making the same.
In the journal "Electrical Design News", Nov. 25, 1981, pp. 141 et seq., a thin-film circuit is described in which a patterned NiCr layer is deposited on a ceramic substrate to form integrated resistors. Disposed over the patterned NiCr layer are a patterned Ni layer as a diffusion barrier and a patterned Au layer for the conductors.
If conventional tin-lead soldered joints are to be made to such gold conductors, the layer thickness of the conductors should not exceed 1.5 .mu.m, because the gold layer is partially dissolved by the molten solder and forms an alloy with the solder. If the percentage of gold in the alloy exceeds a given limit, the alloy becomes brittle. The percentage of gold increases with increasing layer thickness of the gold conductors. Mechanical stress, which may be caused, for example, by temperature changes, tends to result in the formation of cracks which originate from the brittle lead-tin-gold alloy. The risk of cracking increases with increasing layer thickness of the gold conductors.
If a thin-film circuit is to be suitable for use in the GHz range, only conductors with a very low sheet resistivity (R.sub.F .ltoreq.10m ) can be employed.
Since the width of the conductors strongly influences the capacitance between two adjacent conductors, the sheet resistivity is advantageously reduced by increasing the layer thickness, the required layer thicknesses ranging from 6 .mu.m to 10 .mu.m. Gold conductors of this layer thickness which are soldered with lead-tin solders exhibit no sufficient mechanical strength.